One of a number of possibilities of measuring the humidity of a medium is the application of sensing elements by means of which the degree of humidity is expressed by an electric signal. Sensing elements with lithium chloride (LiCl), applied with a suitable binding agent on a support, sensing elements with powdered carbon applied with a suitable binding agent on a support, or sensing elements made on the basis of aluminum oxide (Al.sub.2 O.sub.3) are at present particularly manufactured.
Sensing elements with lithium chloride (LiCl), and those with powdered carbon are sensitive to humidity due to changes of the volume of the layers. Their drawbacks are, however, their slow response to changes of humidity, their unstable calibration curves, and their high dependence on the temperature of the sensing element.
A sensing element on the basis of porous aluminum oxide (Al.sub.2 O.sub.3) has more favorable properties, as water is absorbed on the surface of pores therein, and thus changes of its electrical properties take place. In this case, we have a surface and not a volume process, and thus the time of response of these sensing elements is therefore shorter. The achievable long-term stability and the resistance of these sensing elements depend on the technology of preparation of the porous layer of aluminum oxide (Al.sub.2 O.sub.3) and on the subsequent treatment thereof. At present the best results are achieved with sensing elements made according to the Stover process (a process patented by Stover, U.S. Pat. No. 3,075,385, issued Jan. 29, 1963), in which the layer of aluminum oxide (Al.sub.2 O.sub.3) is obtained by anodic oxidation of a clean polished aluminum surface by alternating current of a density about 12 mA/cm.sup.3 using an electrolyte of 50% sulphuric acid (H.sub.2 SO.sub.4 p.a.) at a temperature of 33.degree. C. for 25 minutes. After rinsing by distilled water, the thus obtained layer is exposed immediately to so-called sealing, which involves immersion in boiling redistilled water for about 30 minutes, after which there is applied on its surface a semi-pervious upper electrode by a suitable method, for instance by vacuum evaporation coating.
A drawback of a thus manufactured sensing element is that in the course of its use the absorbed water acts on the aluminum oxide, causing its gradual hydration, and thus non-reversible changes of the structure of the sensing element and of its properties. These drawbacks can be reduced by passivation of the electric humidity sensing element. The term "passivation" herein used means a specific treatment of the sealed layer of porous aluminum oxide, which treatment prevents its further gradual hydration, without causing any substantial decrease in the porosity of such layer and without destroying its sensitivity to the adsorption of water vapor. The passivation is performed so that the surface of the porous layer of aluminum oxide (Al.sub.2 O.sub.3) is soaked in an aqueous solution of 0.01 to 2 mol of suitable material, for instance sodium dihydrogen orthophosphate (NaH.sub.2 PO.sub.4.2H.sub.2 O), or silicic acid, at a temperature of 35.degree. to 100.degree. C. for 10 to 60 minutes, in accordance with the Czechoslovakian inventor's certificates Nos. 196,880 and 197,742 to Emmer et al.